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Power converters are electrical devices that are used to convert one voltage to another voltage. In most instances, they take a variable voltage and convert that voltage to a fixed voltage. In the case of the topologies discussed herein, the converters do this buy varying the duty cycle of the power switches. If the input voltage decreases, the pulse width, which is controlled by the control circuitry (not shown) increases the ON time of the transistor switches so that the output remains at the desired voltage.
One of the topologies used in converting power from one voltage to another is the two-switch forward converter. In this topology, a primary side voltage is applied across a winding of a transformer and, through that, a voltage is induced in the secondary of the transformer. The voltage across the primary is applied and removed by cycling switches. The alternating application and removal of the primary voltage causes an AC voltage to appear on the secondary of the transformer. This voltage is rectified, filtered, and applied to a load across the output.
When the switches are on, the voltage across the primary of the transformer leads to a build up of magnetizing current within the transformer. This build up of the magnetizing current, if left unchecked, would saturate the transformer, resulting in a decrease of the primary inductance of the transformer and the eventual failure of the circuit due to excessive current. To prevent this failure, the switches are cycled with a duty-cycle that is limited to 50% or less. During the OFF portion of the cycle, the inductive current built up during the ON part of the cycle is dissipated, by returning the energy to the input source. This cycling creates an equal and opposite voltage-time integral across the primary, while limiting the voltage seen by the switches to the input voltage. When the voltage-time integral for each cycle is zero, there is no net increase of the magnetizing current and hence no saturation of the transformer and no reduction of the inductance of the transformer primary, therefore stable operation is possible.
For the forward converter, the 50% duty-cycle limit imposes undesirable limitations on the output inductor, voltage range, transformers, and downstream converters. The output inductor must be larger than in circuits having higher duty-cycles to achieve the same minimum load continuous current in the inductor. The transformer must be able to handle higher RMS current and peak currents for the lower duty-cycle. Therefore, there is impetus to increase the duty-cycle.
One way to increase the duty-cycle has been to use a single-switch converter. In some versions, this variant allows duty-cycles of greater than 75%. However, the single-switch converter requires a way to absorb the magnetizing current thereby resetting the transformer. A snubber/clamp circuit has been used for this, but generates both heat and electrical noise in the form of EMI. A separate reset winding in the transformer has been used also, but increases the cost due to the special transformer and, because of leakage inductance, may not work as well as desired. Because the single-switch converter with a duty cycle greater than 50% places voltages that are more than twice the input voltage across the semiconductors for the off time of the duty-cycles, the semiconductors must be rated for a higher voltage than those used in the two-switch converter. A circuit configuration that increases the duty-cycle above 50% while permitting the use of conventional lower voltage components is needed.
A forward converter implemented with three or more switches allows a transformer to reset more quickly permitting duty-cycles greater than 50% for converters implemented with non-high voltage components. The multi-switch converter uses a transformer in which the primary is implemented in segments with the ends of the segments accessible. The switches bridge the segments and the connections between the primary and input power allowing current flow through the primary when all switches are ON. When all switches are OFF, each segment is separate. Diodes connecting the segments ends and the power rails permit resetting current flow when the switches are OFF. When the switches are ON, the voltage across the entire primary is approximately the input voltage. When the switches are OFF and the diodes are allowing the magnetizing current to flow, each segment of the primary has approximately the input voltage across it. The equivalent of multiple times the input voltage is present across the primary while no more than the input voltage appears across any component. The increased effective primary reset voltage allows a faster reset time during a shortened OFF cycle.
The normal topology of power transformers, implementing the primary winding as two segments with the secondary sandwiched between them, makes the three-switch converter an economical way to implement duty-cycles up to 67%. When higher duty-cycles are required, additional segments, switches and diodes are incorporated in the converter. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.